There is known a DC-DC converter generating a stable output voltage from an input voltage. The DC-DC converter includes a voltage conversion unit generating an output voltage obtained by voltage-converting an input voltage by switching a switching element and a control unit controlling the switching of the switching element for stabilizing the output voltage of the voltage conversion unit.
The configuration of the voltage conversion unit includes a configuration of using a switching element on a high side and using a diode on a low side, a configuration of using switching elements on the high side and the low side, that is, a configuration of using a synchronous rectification method, and the like. In this synchronous rectification method, a switching element having a smaller voltage drop amount than a diode is used also on the low side, and thereby power conversion efficiency can be improved. Patent literature 1 discloses a DC-DC converter using the synchronous rectification method in the voltage conversion unit. In this DC-DC converter using the synchronous rectification method, for preventing short circuit from occurring when the two switching elements are turned on at the same time, there is provided a period when these switching elements are turned off at the same time, that is, a dead time.
Meanwhile, the control means of the control unit includes, for example, a means of using a PWM (pulse width modulation) method, a means of using a comparator method, and the like. The means of using the PWM method keeps the switching frequency of the switching element constant and adjusts an on-pulse width to stabilize the output voltage of the voltage conversion unit. On the other hand, the comparator method keeps the on-pulse width of the switching element constant using a comparator and adjusts an off-pulse width (i.e., switching frequency) to stabilize the output voltage of the voltage conversion unit.
Because of a difference between these control means, the comparator method has an advantage over the PWM method as follows. The DC-DC converter is sometimes used as a voltage source of a PU (Processor Unit) or the like. In the PU, consumption current is increased abruptly when the PU shifts from a waiting state to a processing state. When an output voltage is reduced abruptly by the abrupt increase of a load current, the comparator type DC-DC converter outputs the on-pulse instantly when detecting the reduction of the output voltage by the comparator and thereby stabilizes the output voltage more quickly than the PWM method which cannot output the pulse during a predetermined off-pulse period. In this manner, the comparator method has an advantage over the PWM method in that a response characteristic thereof for the abrupt increase of the load current is better.
This comparator type DC-DC converter is provided with an on-time for determining the fixed on-pulse width and a minimum off-time for determining the minimum off-pulse width. This minimum off-time is provided from the following reason. In the comparator type DC-DC converter, when ON/OFF is switched in the switching element, a power source voltage sometimes varies on a high potential side due to the switching of operation states in a drive circuit for driving the switching element. In particular, when the switching element on the high side is switched from ON to OFF and the switching element on the low side is switched from OFF to ON, that is, when the output voltage is reduced lower than a reference voltage, the comparator is operated. At this time, when a power source voltage and a reference voltage for the comparator vary due to power source voltage variation on the high potential side, there is a possibility that the comparator is operated erroneously. Accordingly, the minimum off-time is provided for preventing the converter from switching to the on-pulse generation, that is, for continuing the off-pulse generation during a predetermined period until the respective variations in the power source voltage on the high potential side and the reference voltage are reduced, even when the comparator is operated erroneously.